While it's been a while, i don't remember there being any money in Duke Nukem 3D outside of this specific instance, and then it was just an animation that popped up when pressing the Use key while looking at a prostitute(generally resulting in them doing a more provocative pose for a moment). By extension, killing them didn't gain you any money, either, since there wasn't any. You could shoot them, but it would just pointlessly waste ammo.
I didn't find the right solution from the Internet.
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